1. Field of the Invention
The invention relates to a wind energy system having a rotor that can be driven by the wind, preferably having one or more rotor blades that can be adjusted in angle, a generator directly or indirectly connected with the rotor, to generate electric energy, which is configured as an asynchronous generator having a super-synchronous converter cascade in the rotor circuit, for slip-variable generator operation ,so that power output of the generator is possible at different speeds of rotation of the rotor, and an operation guide system that is configured to regulate the speed of rotation of the rotor, within a predetermined wind speed range, preferably with adjustment of the rotor blade angles.
Furthermore, the invention relates to a method for regulating the power output of a wind energy system, in that the slip is regulated.
2. The Prior Art
A system of this type is known from the older application of the applicant, DE 10117212.5.
Such wind energy systems, which work at a variable speed of rotation of the rotor and variable rotor blade angles, produce more electric energy than systems that work at a single, fixed speed of rotation of the rotor and fixed, predetermined rotor blade angles. Usually, work at a variable speed of rotation takes place in the range of very low wind speeds, whereby the rotor blades assume a large angle relative to the incident wind, which is only slightly less than 90 degrees. This rotor blade angle is not changed, at first, with an increasing wind speed, until the wind speed is sufficient to turn the rotor at the rated speed of rotation, whereby the wind energy system puts out its rated output. The power output therefore increases, proceeding from a very small output at a minimum wind speed, along with the speed of rotation of the rotor, until the rated power is reached. If the wind speed increases further, the rated power and the rated speed of rotation are now kept constant, as much as possible, until the wind speed increases above a shut-off speed. Here, the wind energy system is shut off, in that the rotor blades are turned completely in the wind direction, so that the rotor blade angles relative to the wind direction amount to approximately zero degrees. In this way, the rotor is braked. Shut-off at very high wind speeds is necessary because the stress on the wind energy system in operation under very strong winds, particularly gusts, can become so great that damage can occur.
With an increasing power and number of such wind energy systems, equalization of short-term power variations in the utilities poses greater and greater difficulties. For this reason, the demands on the quality of the current that is supplied are constantly increasing.
For this purpose, suitable controls for asynchronous generators have been developed. Such an asynchronous generator achieves a variable slip of up to 10%, which means that the rotor and the generator permit a variation of 10% in the speed of rotation during gusts, by means of suitable computer control. In the case of strong winds, the generator constantly holds the electricity generation at the rated power. The elasticity in the system minimizes the stress on the vital components of the wind energy system, and improves the quality of the current that is being fed into the power network.
In the case of this known method of operation of the wind power system having a rotary current generator, the adaptation of the speed of rotation for uniform supply to the network in the case of gusty winds is implemented by means of a changeable slip, with a pulse-controlled resistor in the rotor circuit. The rotor slip power remains unused and is converted into heat.
Another known concept is the super-synchronous cascade for an asynchronous generator having slip rings. In this connection, the rectified slip power is fed into the network by way of a network-guided inverter. Disadvantages in this application are strong pendulum moments of the current oscillations that are produced in the generator, a great need for reactive power, and possible commutation failure of the inverter.
Another method of a cascade, known from U.S. Pat. No. 6,137,187, is regulation of the rotor currents with a voltage intermediate circuit converter, according to a field-oriented method for a dual-fed rotary current generator, for utilization of a supra-synchronous and a super-synchronous range of the speed of rotation. The field-oriented regulation methods of the dual-fed asynchronous generator are problematic in the case of short voltage collapses in the network, and in case of network short-circuit.